1. Field of Invention
The present invention relates generally to an apparatus and method for controlled de-pressurization of a nuclear reactor, and more particularly, to an improved gas sparging system for reducing loads acting on structures submerged in a suppression pool.
2. Discussion
In the event of over-pressurization of a nuclear reactor, relief valves may vent steam or reactor coolant into a suppression pool--a tank filled with liquid coolant--to dissipate the energy of the vented steam. The relief valve's abrupt opening, and subsequent delivery of high-pressure steam to the suppression pool, results in dynamic loads on suppression pool walls and structures. These dynamic loads, if large enough and if not properly accounted for during plant design, can damage structures submerged in the suppression pool.
Dynamic loads within the suppression tank are thought to occur through at least two different mechanisms. In a typical pressure relief system, a relief valve exhausts high pressure steam into a discharge line, which is connected to a group of gas spargers. The spargers generally consist of vertical pipes whose ends are submerged in the suppression pool. When the pressure relief valve vents high pressure steam into the exhaust line, the steam must first displace noncondensable gas and liquid coolant present in the sparger pipe. During this sparger line clearing process, the high pressure steam compresses the noncondensable gas because of the relatively large inertia and high flow resistance of the liquid coolant. As the compressed noncondensable gas emerges from the sparger nozzles, it expands rapidly and then contracts due to over expansion. The expansion and contraction of the noncondensable gas repeats during the line clearing process, resulting in oscillatory pressure waves that impact submerged structures within the suppression pool.
At some point after the liquid coolant has cleared the sparger pipe, the sparger injects high pressure steam into the suppression pool, creating a vapor-phase injection zone adjacent to the sparger nozzles (in practice there appears to be no clear transition between non-condensable gas venting and steam venting). Because of time-dependent imbalances between the steam mass flux and condensation rate, the high pressure steam injection process results in pressure oscillations. Like the line clearing process, oscillatory pressure waves during steam injection give rise to dynamic pressure loads on submerged structures within the suppression pool.
In many conventional pressure relief systems, the gas spargers simultaneously exhaust steam into the liquid coolant at different locations, which distributes pressure forces acting on submerged structures within the suppression pool. But, dynamic loads on submerged structures can still be large because pressure disturbances from different spargers can combine. For example, if pressure disturbances from two adjacent spargers have the same frequency and phase relationship, the amplitude of the two pressure disturbances will add, resulting in a combined pressure disturbance that is greater than the individual pressure disturbances. Thus, pressure relief systems that take into account the interaction of pressure disturbances from individual spargers in order to minimize dynamic loads on structures within the suppression pool would be desirable.